The present invention will be described specifically in connection with dental radiology and its use in determining on-going structural changes in teeth and related structures (such as remineralization). However, it will be understood from what follows that the principles of this invention could be applied in other areas.
It is known to utilize a computer to carry out densitometric analyses of standardized radiographs taken of the same tissue structure at different points in time, in order to detect tissue change. For example, in the dental area it is known to compare radiographs of the same tooth structure taken at different times, in order to detect remineralization in the teeth or lesions in the periodontal tissue. One technique utilized in this procedure is called subtraction radiography, wherein a computer will digitize two standardized radiographs of the same structure taken at different times, and then subtract the one from the other. If the two radiographs are identical, the result will be a uniform picture with no apparent detail. However, if the radiographs differ, for example at anatomical locations which have changed between the exposures, then the subtraction image will exhibit a different density (grey level) than the background. The above principles are well understood, and form part of the prior art. Reference may be made to an article entitled "Subtraction Radiography and Computer Assisted Densitometric Analyses of Standardized Radiographs", by Ortman et al, published in the Journal of Periodontal Research, 1985: 20:644-651.
Thus, it will be understood that, when wishing to compare radiographic images taken at different times of the same dental structure for the purpose of following changes in the structure, the ideal situation is that in which both or all of the images are made under precisely the same conditions of exposure. In other words, for every image, the relative positions of the film, the teeth and the source of X-rays would be identical. Moreover, the degree of exposure, the film characteristics, and the intensity of the X-ray source would also have to be the same from exposure to exposure. If this could be achieved consistently, then subtraction radiography could be carried out directly and the information thus generated would be highly reliable.
However, as a practical matter the perfection of alignment is generally not attained. Although a bite block of conventional nature can be utilized to ensure to a large degree that the position of the film with respect to the teeth is consistent from image to image, it will be appreciated that even a slight change in the geometric position of the source of the X-rays from image to image (further or closer, up or down, right or left) will significantly alter the scale or position of the image by an amount sufficient to interfere with analysis of structural changes by the technique of subtraction radiography. Furthermore, where the film is improperly inserted into the bite block, sequential images can be mutually rotated even where the source of the X-ray is precisely the same.
Conventionally, attempts have been made to overcome the alignment problem by utilizing external beam-image receptor alignment devices that are very cumbersome and problematic to use. Reference may be had to an article by McHenry et al, entitled "Methodological Aspects and Quantitative Adjuncts to Computerized Subtraction Radiography", published in the Journal of Periodontal Research 1987; 22: 125-132. Even with such devices, however, problems can persist in attaining perfect alignment. For example, problems arising through improper film insertion into a bite block are not overcome by the external alignment devices. And of course, such devices do not address the problem of attaining gray-scale uniformity.